Confinement of a Currentless Plasma in the Heliotron-E

Abstract

In the toroidal plasma confinement devices using external helical conductors for forming the nested magnetic surfaces, such as heliotrons, torsatrons, and stellarators, the plasma equilibrium can be maintained without Ohmic-heating current. This property can, in principle, lead to the steady-state operation of a controlled thermonuclear reactor, and has led to recent efforts using injected energetic neutral beams 1 ' 2 and radio-frequency waves 2 * 3 to sustain the plasma in stellarators and heliotrons. A nearly currentfree plasma was recently reported 1 in the Wendelstein VII-A stellarator in which substantial ion heating was observed by neutral-beam injection. The nature of energy confinement in the experiment, however, has not been fully resolved because of the dominance of radiative loss in the experiment.This paper presents experimental results on confinement of a plasma initiated and heated by radio-frequency waves at the electron cyclotronresonance frequency in a heliotron configuration. In the afterglow regime the radiative loss is measured to be small and the energy loss rate of electrons is dominated by equipartition with ions. This allows a comparison with neoclassical theory.The Helotron-E device (major radius R = 2.2 m, average minor radius of plasma r p =0.2 m) is an asymmetric toroidal system with large rotational transform [I(0)/2TT = 0.5, i{r p )/2u = 2.2] and strong shear. The Z = 2 helical coils can produce a maximum magnetic field of 20 kG. The experiment was performed at 10 kG to match the electron cyclotron-resonance heating (ECRH) power available at 28 GHz. The microwave tube has a peak output power of 200 kW and pulse length of 10 msec. The cutoff density is 1.0 x 10 13 cm" 3 for the ordinary wave. The microwave output was in TE 02 circular mode, transmitted to the Heliotron-E by a 2.5-in.-diam waveguide. The radiated power consists of an almost equal mixture of ordinary and extraordinary waves. As shown in Fig. 1, the wave was launched from the low-field side. The ordinary wave was expected to be transmitted directly into the plasma and the extraordinary wave would be reflected at the righthand cutoff. It can be expected, however, that Confinement of a plasma initiated and heated by electron cyclotron-resonance-frequency waves has been studied in the Heliotron-E. The observed plasma parameters were T e * 500-100 eV, T t ^ 100 eV, n e = 4x lo 12 cm" 3 at B = 10 kG. The electron energy decay time was 40± 8 msec and the particle confinement time was estimated to be more than 70 msec. These results offer a considerable improvement in energy confinement over Ohmically heated plasmas.

Neutral-beam injection heating experiment on currentless plasma in the heliotron E device

“…During the last few years, substantial progress has been made in experiments with helical systems. Special effort has been concentrated on currentless plasma production [1][2][3][4][5][6][7] and subsequent heating of such a plasma. The energy confinement times essentially follow a drift parameter scaling for Joule-heated discharges.…”

Pressure-driven relaxation instability in a current-free high-shear helical system

“…The W VII-A stellarator succeeded in obtaining currentless high-density plasmas by reducing the Ohmic-heating current during neutral-beam injection (NBI) [9]. On the other hand, Heliotron E obtained currentless plasmas by injecting the neutral beam into electron-cyclotron-resonance-heated (ECRH) plasmas with a gyrotron frequency of f = 53.2 GHz [10]. Heliotron E also produced highbeta currentless plasmas by reducing the magnetic field strength at the magnetic axis to 9.4 kG, with application of second-harmonic heating for target plasma production.…”

Effects of magnetic shear on electron cyclotron resonance heating in heliotron/torsatron configurations

“…4 In helical systems, especially, ECRH is indispensable for producing high temperature currentless plasmas without Ohmic heating. 5 The ECRH power should be absorbed in a single pass, for obtaining a localized power deposition and measuring the thermal transport accurately. So far, the launching conditions, such as injection angle and polarization, have been determined by the magnetic field at the plasma surface where the waves are coupled to propagating modes such as the ordinary ͑O͒ and extraordinary ͑X͒ modes.…”